This application claims the priority of German application No. 100 45 121.7, filed Sep. 13, 2000, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a method for obtaining gaseous products by the low-temperature fractionation of air. The method includes (1) supplying a first, purified, and cooled stream of air to the high-pressure column; (2) passing at least one liquid stream from the high-pressure column into the low-pressure column; (3) drawing off a product stream in the liquid state from the low-pressure column and, in the liquid state, bringing the product stream to an elevated pressure; (4) evaporating the product stream, under the elevated pressure, in an indirect heat exchange with a second purified stream of air, which is condensed at least partly during the indirect heat exchange; and (5) work-expanding at least part of the second stream of air and subsequently passing the second stream of air into the low-pressure column.
The product stream, which is evaporated by a portion of the air (the second air stream), preferably is an oxygen product from the lower region of the low-pressure column of any purity (for example, 90 to 99.8% and preferably 98 to 99.9%). Preferred areas of application of the present invention are methods in which the second air stream, which is used to evaporate the product stream, has a pressure that is only slightly if at all higher than the operating pressure of the high pressure column (for example, up to twice the pressure of the high pressure column). In this case, all pressure are clearly in the non-critical range; the concepts of xe2x80x9cevaporatingxe2x80x9d and xe2x80x9ccondensingxe2x80x9d are to be understood in this connection as a phase transition. If oxygen is evaporated under such a relatively low pressure, this step of the process is usually not carried out in a main heat exchanger, which is used to cool the air used from ambient temperature to the rectifying temperature. Instead, this step of the process is carried out in a separate secondary condenser. A liquid cycle with rinsing can be set up there, which prevents operating and safety problems resulting from the deposition of components of low volatility.
In addition, the present invention can, in principle, also be used at higher product pressures, which may even be above the critical pressure. In this connection, the concepts of xe2x80x9cevaporatingxe2x80x9d and xe2x80x9ccondensingxe2x80x9d also include xe2x80x9cpseudo-evaporatingxe2x80x9d and xe2x80x9cpseudo-condensingxe2x80x9d. Such a method is known from the EP 869322 A1 (FIG. 3). The pressure, to which liquid or supercritical air is subjected, is relieved in two steps and performs work. Initially, it is relieved in a first step to about the pressure of the high-pressure column and subsequently partially further in a second step to the pressure of the low-pressure column.
It is an aspect of the present invention to provide a method of the type given above, and a corresponding apparatus, which are particularly economically advantageous.
This aspect is accomplished due to the work-expanding of at least part of the second air stream being carried out in a single step. As a result, the pressure difference between the condensation pressure of the second air stream and the pressure of the low-pressure column is utilized particularly efficiently with simple equipment.
The work expansion is carried out in a turbine, which is coupled to a braking device. The braking device may be, for example, a generator or an oil brake.
According to an embodiment of the present invention, it is advantageous if a third air stream is cooled to an intermediate temperature between ambient temperature and the rectifying temperature. This stream of air is expanded while producing work, and the stream of air is supplied to the low-pressure column. Therefore, in addition to the condensed, second stream of air, a further gaseous stream of air is introduced directly into the low-pressure column.
With the help of the two work-performing expansion steps carried out (second and third streams of air), the xe2x80x9cnaturalxe2x80x9d pressure drop between the high-pressure column and the low-pressure column is utilized optimally. In many cases, it is possible to recover the whole of the abstracted heat, required for the method, without consuming external energy for compressing air to a pressure clearly above the operating pressure of the high-pressure column. The work expansion machine for the third stream of air is also coupled with a braking device, preferably a generator or a secondary compressor. The secondary compressor can be used, for example, for the secondary compression of the second stream of air, which is used to evaporate the product stream. This secondary compression can take place in the hot or in the cold.
The work-performing expanded second stream of air can be introduced completely or partly directly into the low-pressure column. In many methods, the nitrogen-oxygen fractionation in the high-pressure column and the low-pressure column is followed by the recovery of argon. For this purpose, an argon-containing fraction from the low-pressure column is supplied to a crude argon rectification. In this case, it is advantageous to pass the work-performing expanded second stream of air, before it is introduced into the low-pressure column, into the evaporation space of the condenser-evaporator, which is used for producing liquid reflux for the crude argon rectification and can be constructed, for example, as a head condenser.
The present invention is particularly advantageous at moderate product pressures in the product stream, which is to be evaporated. In such cases, the pressure of the second air stream during the indirect heat exchange with the evaporating product stream is, for example, not greater than 1.5 times the operating pressure in the sump of the high-pressure column. In this connection, it is advantageous if the indirect heat exchange for evaporating the product stream in the liquid state is carried out in a secondary condenser, which is separate from a main heat exchanger, in which the first stream of air is cooled. After it is evaporated in the secondary condenser, the product stream can be introduced into the main heat exchanger and heated there.
Preferably, the first stream of air and the second stream of air and, optionally, the third stream of air are compressed jointly to approximately the operating pressure of the high-pressure column. As a result, the cost of the equipment for compressing the air remains relatively low. If necessary, the second stream of air can be compressed further, warm or cold, downstream from this joint compression.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.